In the past, there have been developed multi-fuel internal combustion engines which are able to perform mixed combustion of a plurality of kinds of fuels such as a liquid fuel (e.g., light oil, gasoline, etc.) and a gas fuel (e.g., compressed natural gas, hydrogen gas, etc.).
For example, in Patent Document 1, there is disclosed a compression ignition type internal combustion engine which uses, as a main fuel, a gas fuel such as natural gas, propane, biogas, landfill gas, or hydrogen gas, etc., and as a pilot fuel, diesel fuel or dimethyl ether which self-ignites more easily than the main fuel. In the compression ignition type internal combustion engine described in Patent Document 1, the main fuel is injected into a cylinder in the intake stroke or in the early stage of the compression stroke, so that a pre-mixed gas is formed therein by the main fuel and intake air. At this time, the amount of the main fuel injected into the cylinder is limited in order to prevent knocking. Moreover, during the compression stroke, the pilot fuel is injected into the cylinder, and an additional amount of main fuel is injected into the cylinder in the vicinity of top dead center, in order to fulfill the load requirement of the engine.
In each of Patent Document 2 and Patent Document 3, there is disclosed a compression ignition type internal combustion engine using a liquid fuel and a gas fuel. In the compression ignition type internal combustion engine described in each of the second and third patent documents, the gas fuel is injected into an intake port from a gas injector. In addition, the timing for injecting the gas fuel from the gas injector is set between a point in time at which an exhaust valve was closed and a point in time at which an intake valve is closed, in the intake stroke.
In Patent Document 4, there is disclosed a compression ignition type internal combustion engine using light oil and natural gas. In addition, in Patent Document 4, there is also disclosed a system which controls the amount of natural gas to be supplied to the internal combustion engine according to the engine load of the internal combustion engine.